1. Field of the Invention
This invention relates generally to telecommunication systems and more particularly, it relates to an impedance blocking filter circuit used in telecommunication systems for interconnecting between incoming telephone lines from a telephone company's central office (C.O.) and subscriber or customer telephone equipment such as a telephone set located at a subscriber's premises so as to unconditionally block telephone impedance above 20 KHz.
2. Description of the Prior Art
The prior art appears to be best exemplified in the following U.S. Letters Patent which were developed in a search directed to the subject matter in this application:
4,613,732 4,823,383 4,742,541 5,642,416 4,743,999 5,802,170
In U.S. Pat. No. 4,823,383 issued to Cardot et al. on Apr. 18, 1989, there is disclosed a protection device for terminal equipment on telephone subscriber premises which includes a voltage surge protection circuit and/or a filter for providing protection against radio frequencies and interference. The filter is comprised of series inductors L1, L2, L3 and L5 interconnected between terminals E1 and S1 and series inductors L'1, L'2, L4 and L'5 interconnected between terminals E2 and S2. A capacitor C5 is connected between the junctions of the inductors L2, L3 and the inductors L'2, L4. The surge protection circuit includes thermistors TH1, TH2 and voltage limiters D1-D3.
In U.S. Pat. No. 5,802,170 issued to Smith et al. on Sep. 1, 1998, there is disclosed a customer bridge module for connecting telephone company wiring and subscriber telephone wiring in a telephone network interface apparatus. In one embodiment, the customer bridge module includes overcurrent protection and an RFI filter. The overcurrent protection is formed by positive temperature coefficient resistors 220, 222 and inductors. The RFI filter is formed by inductors 224a-224c, 226a-226c and capacitors 236a-236c. The inductors and capacitors are used to form a multi-pole low pass filter.
In U.S. Pat. No. 5,642,416 issued to Hill et al. on Jun. 24, 1997, there is disclosed an electromagnetic interference by-pass filter which suppresses RF noise currents conducted over the tip and ring leads of a telephone line-powered instrument. The filter includes first and second inductors 51, 53 and first and second capacitors 41, 43.
It is generally well-known these days that many telephone subscribers or customers also have a personal computer located on their premises. At times, the computer user receives ADSL (an acronym for Asymmetric Digital Subscriber Line) signals from the Internet over the same telephone lines via an Internet Server Provider (ISP). In order to increase the speed of downloading of information from the Internet, an ADSL network interface is typically purchased and installed between the incoming telephone lines and the user's computer. However, since one or more telephone subscriber terminal equipment such as telephone sets, facsimile machines and/or answering devices are also connected to the same incoming telephone lines via internal house wiring, ADSL interference problems may be caused by the terminal equipment which can significantly limit or reduce the data rate. In one situation, it has been experienced that the change of state from "on-hook" to "off-hook" of the telephone equipment and sometimes the telephone terminal equipment even being "on-hook" can create a resonance effect to occur so as to drop the impedance value to less than 10 .OMEGA.(Ohms) at a frequency as high as 500 KHz.
Accordingly, it would be desirable to provide an impedance blocking filter circuit for connection to the telephone terminal equipment causing the erratic input impedances. The impedance blocking filter circuit of the present invention is of a modular design so as to be easily connected in series with the offending telephone terminal equipment. The impedance blocking filter circuit blocks unconditionally any telephone impedances (e.g., open, short, capacitive, inductive, resonant, or any combination thereof) above the frequency of 20 KHz.